The present invention relates generally to batch ion implantation systems, and more particularly to a method and system for determining pressure compensation factors in an ion implanter.
In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion beam implanters are used to treat silicon wafers with an ion beam, in order to produce n or p type extrinsic material doping or to form passivation layers during fabrication of an integrated circuit. When used for doping semiconductors, the ion beam implanter injects a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in xe2x80x9cn typexe2x80x9d extrinsic material wafers, whereas if xe2x80x9cp typexe2x80x9d extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium may be implanted.
Typical ion beam implanters include an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and directed along a predetermined beam path to an implantation station. The ion beam implanter may include beam forming and shaping structures extending between the ion source and the implantation station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the beam passes en route to the implantation station. When operating an implanter, this passageway is typically evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
The mass of an ion relative to the charge thereon (e.g., charge-to-mass ratio) affects the degree to which it is accelerated both axially and transversely by an electrostatic or magnetic field. Therefore, the beam which reaches a desired area of a semiconductor wafer or other target can be made very pure since ions of undesirable molecular weight will be deflected to positions away from the beam and implantation of other than desired materials can be avoided. The process of selectively separating ions of desired and undesired charge-to-mass ratios is known as mass analysis. Mass analyzers typically employ a mass analysis magnet creating a dipole magnetic field to deflect various ions in an ion beam via magnetic deflection in an arcuate passageway which will effectively separate ions of different charge-to-mass ratios.
Dosimetry is the measurement of ions implanted in a wafer or other workpiece. In controlling the dosage of implanted ions, closed loop feedback control systems typically are utilized in order to dynamically adjust the implantation to achieve uniformity in the implanted workpiece. Such control systems utilize real-time current monitoring to control the slow scan velocity. A Faraday disk or Faraday cup periodically measures the beam current and adjusts the slow scan speed to ensure constant dosing. Frequent measurement allows the dose control system to respond quickly to changes in beam current. The Faraday cup may be stationary, well shielded, and located close to the wafers, making it sensitive to the beam current actually dosing the wafers.
Faraday cups measure only the electric current. Interactions between the ion beam and gases evolved during implant can cause the electric current, a charge flux, to vary even when the particle current, a dopant flux, is constant. To compensate for this effect, the dose controller reads the beam current from the Faraday cup and the pressure from a pressure gauge simultaneously. When a pressure compensation factor (PCOMP) is specified for an implantation recipe, the measured beam current is modified by software to present a compensated beam current signal to the circuit controlling the slow scan. The amount of compensation (e.g., in the compensated beam current signal) in such a closed loop system may thus be a function of both the pressure and the pressure compensation factor.
When properly applied, pressure compensation improves repeatability and uniformity over a wide range of implant pressures. However, the vacuum in an implanter is never perfect. There is always some residual gas in the system. Usually the residual gas poses no problems (in fact, a small amount of residual gas is needed for good beam transport and effective charge control). However, at high enough pressure, charge exchange between the ion beam and the residual gas can cause dosimetry errors. If the shift in dose between implants into bare wafers and implants into photoresist-coated (PR) wafers is unacceptably large, or if the dose uniformity is significantly degraded, then pressure compensation may be employed in order to ensure uniformity.
Charge exchange reactions between ion beams and residual gas can add or subtract electrons from the ion, changing the ion""s charge state from the value stated in the recipe. When the charge exchange reaction is neutralization, a portion of the incident ion flux is neutralized. The result is a reduction in the electrical current while the particle current (including neutrals) remains unchanged. When the charge exchange reaction is electron stripping, a portion of the ion flux loses electrons. The result is an increase in the electrical current while the particle current remains the same.
For typical recipes where charge exchange is an issue, the beam undergoes much more neutralization than stripping. As a result, the beam current measured by the Faraday cup decreases whenever the end station pressure increases. Ions in the beam are neutralized, but they are not deflected or stopped by residual gas. The dose rate, dopant atoms per area per time, is unchanged by charge exchange after the analyzer magnet. Implanted neutrals contribute to the dose received by the wafer, but are not measured by the Faraday cup. As a result, the wafer is overdosed. Without pressure compensation, charge exchange neutralization can limit the dose uniformity and repeatability.
Pressure Compensation may thus be employed whenever charge exchange between the ion beam and residual gas in the process chamber has a significant effect on dose. The pressure at which this happens depends on the recipe and the process specification. For some recipes, compensation is required to meet implanter specification when the pressure due to photoresist outgassing is 5xc3x9710xe2x88x926 torr as measured on the pressure gauge. For recipes where the pressure due to photoresist outgassing is 2xc3x9710xe2x88x925 torr or higher, compensation is worth investigating. Such compensation may include measuring the effect of photoresist outgassing by implanting monitor wafers with and without photoresist, and comparing the measured variation to the process specification. The amount of compensation required depends on the pressure, which the dose controller reads from the pressure gauge during the implant. It also depends on a recipe-selectable parameter PCOMP that accounts for other factors.
Some of these factors depend on the hardware configuration (size and number of pumps, location of pumps, location of ion gauge); other factors depend on the recipe: ion species, charge state, and energy. The dependence of PCOMP on energy is weak in the range from 20 keV to 90 keV. In general, however, PCOMP should be determined for each recipe and implanter. The parameters necessary for pressure compensation have traditionally been determined by implanting a matrix of at least four test wafers at non-standard operating conditions. However, test wafers are expensive. In addition, the implantation and measurement of such test wafers requires an implantation system to be taken off line, resulting in lost production and other costs. Thus, there is a need for improved methods and systems for determining pressure compensation factors in ion implanters.
The present invention is directed to a method and system for determining a pressure compensation factor for use in an ion implantation system. In accordance with one aspect of the present invention, the method comprises providing a test workpiece in the ion implantation system, the test workpiece having at least one band region, assuming an initial predicted pressure compensation factor. The method further comprises implanting the at least one band region of the test workpiece with an ion beam using the ion implantation system and the initial predicted pressure compensation factor while measuring ion beam current and a pressure in the ion implantation system. A sheet resistance associated with the implanted test workpiece is then measured, and a pressure compensation factor is determined according to the initial predicted pressure compensation factor, the measured sheet resistance, the measured ion beam current, the measured pressure, and a desired sheet resistance. The method thus provides a compensation factor while using only one test workpiece or wafer, via the employment of measured pressure information.
In order to further refine the compensation factor determination, first and second test workpieces may be provided in the ion implantation system, wherein the first and second test workpieces each have at least one band region, and wherein one of the first and second test workpieces includes a photoresist, and the other of the first and second test workpieces is bare or a blank wafer. The method further comprises assuming an initial predicted pressure compensation factor, implanting the at least one band region of the first and second test workpieces with an ion beam using the ion implantation system and the initial predicted pressure compensation factor while measuring ion beam current and a pressure in the ion implantation system. First and second sheet resistances associated with the implanted first and second test workpieces are then measured, respectively, and a pressure compensation factor is determined according to the initial predicted pressure compensation factor, the first and second measured sheet resistances, the measured ion beam current, the measured pressure and a desired sheet resistance.
In accordance with another aspect of the invention, there is provided a system for determining a pressure compensation factor for use in an ion implantation system, which comprises a test workpiece having at least one band region, and a means for implanting the at least one band region of the test workpiece with an ion beam using the ion implantation system and an initial predicted pressure compensation factor. The system further comprises means for measuring an ion beam current and a pressure in the ion implantation system, and a means for measuring a sheet resistance associated with the implanted test workpiece. Lastly, the system further comprises a means for determining a pressure compensation factor according to the initial predicted pressure compensation factor, the measured sheet resistance, the measured ion beam current, the measured pressure, and a desired sheet resistance.